This article marks the first of a three-part series on water issues in
the Southwest. In this part, we will review the primary ground and surface resources
available to our growing population. In the next two parts, we will examine 1)
the problems emerging from a dwindling supply, growing demand and potential
long-term drought and 2) the potential long-term solutions for looming water
shortages.
During the drought we have experienced over the past several years across
much of the desert Southwest – including southern California, southern
Nevada, southern Utah, southwestern Colorado, Arizona, New Mexico and western
Texas – our water tables have fallen, wetlands have diminished, stream
flows have dwindled and lakes have contracted.
At the same time, our population has continued to grow rapidly, increasing
the demand for water. Global warming has intensified, delaying autumn snowfalls
and accelerating spring snow melts, raising autumn and spring temperatures, and
drying out soils. Drought-stressed wood- and brushlands have been dying off,
becoming fuel for massive wildfires. Forests have been retreating up mountain
slopes, taking sanctuary in the wetter zones. From the Pacific Coast eastward
to the High Plains, Southwest communities – for instance, San Diego, Las
Vegas, Tucson, Albuquerque and El Paso – confront potential shortages and
must routinely impose water use restrictions.
Some scientists have predicted that we can expect the drought to continue,
bringing to the Southwest a dust bowl similar to the one the Southern Plains
experienced during the Great Depression of the 1930s. Columbia University's Lamont-Doherty
Earth Observatory scientists suspect that we may have already entered a long-term
extreme drought.
Under the specter of a potential prolonged severe drought – perhaps
much like those that have occurred in the Southwest more than a dozen times during
the past eight centuries, according to the U.S. Geological Survey – federal,
state and local government agencies have broadened efforts to reassess our region's
water resources and consumption, identify and gauge potential problems, and develop
innovative new management and technical solutions. It is the declining resources – both
ground and surface waters – that signal change for our future.
Water Resources
Assessing our usable ground and surface water resources can be notoriously
difficult. Aquifers, for instance, occur in widely scattered basin locations
and at variable depths across the Southwest. Their waters vary in quality and
accessibility. Their recharge processes and rates defy ready understanding. Stream
flows and lake levels rise and fall largely in response to the amount and timing
of snow and rain in the higher elevations, the onset of snow-melting temperatures
in the spring, the locations and timing of the typically intense but scattered
rainstorms of the Southwest, and the changeable needs for water drawn from ground
or from surface resources. The resources, which largely define our lives in the
Southwest, raise major challenges for water managers and technical experts.
Ground Water
From the eastern escarpments of the Southern Plains westward across the Chihuahuan,
Sonoran and Mojave Deserts, our ground water resources – or aquifers, or
bolsons – reside within the sedimentary layers that fill the thinly vegetated
basins between the mountain ranges. Originally, as the USGS said in its Internet
site, Science for a Changing World, "Desert Basins of the Southwest," the
ground water became accessible for human consumption when it issued from an aquifer
as a spring or a seep, forming a stream or a wetland. Later, shallow ground water
became available through hand-dug wells or through low-capacity drilled wells.
More recently, the deeper ground water has been tapped by "high-capacity
large-diameter drilled wells with turbine pumps…"
Most water flowing into a typical Southwest aquifer originates primarily as
rainfall and snowmelt. Most of it flows from higher – and wetter – elevations
through subsurface structural fractures or through surface streambeds into the
margins of the basins. It seeps through the sedimentary layers – gravels,
sands, silts and clays – to collect in the aquifers, sometimes moving at
a rate of no more than several dozen feet in the course of an entire year. Gradually,
it saturates the minute spaces, or "pores," between the grains of sedimentary
layers below the water table. While sedimentary layers may approach two
miles in depth in some of our Southwest basins, the most accessible and highest
quality water trapped within the layers typically lies within the upper few thousand
feet. Eventually, sometimes after tens of thousands of years in an aquifer, water
may reemerge as a spring, join with a neighboring aquifer, seep into wetlands
or marshes, or bring nourishment to desert plants.
Over time, even in our dry Southwest, large volumes of water may accumulate
in a single aquifer. "For example," said the USGS, "in a basin
in which the upper 1,000 feet of saturated sediments contain 15 percent removable
water and having an average width and length of 15 miles and 25 miles respectively,
the volume of water is 36 million acre-feet. [An acre-foot – a volume of
water that would cover an acre of land to a foot in depth – equals about
326,000 gallons.] That volume of water [36 million acre-feet], if it could be
practically pumped, would amount to a 2,000-year supply for a population of 100,000
having a per capita water use of 160 gallons per day." Collectively, prodigious
volumes of water have collected in the Southwest's numerous aquifers.
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